HM Revenue and Customs will shortly be sending out letters to families informing them they will see their child benefit if one parent earns over £50,000 a year. Families where a parent earns over £60,000 will lose the money completely.

Experts have warned that most middle-class families remain oblivious to the changes, which will require an estimated 500,000 people to fill out complicated self-assessment tax returns for the first time.

There are also fears that it will produce strange loopholes for divorced couples and those where one parent earns just below the threshold.

The Government was also criticised last night for handing child benefit to the parents of more than 40,000 children living abroad, while preparing cut the payments for a million British households.

George Osborne, the Chancellor, is coming under pressure to justify the £36 million bill for foreign children, as the Treasury takes an axe to the hand-outs worth up to £1,000 per year for higher-earning UK residents.

Most of the 23,000 claimants for overseas children are foreigners working in the UK, with around two-thirds sending the cash back to children in Poland.

The Treasury says it is forced to pay for the children of any UK workers who contribute National Insurance under European Union rules.

However, Priti Patel, MP for Witham, who uncovered the figures, said the excuse is not good enough.

She said the loophole that allows immigrants to “abuse our benefits system” must be shut down.

A Treasury spokesman defended the payment of benefits to children living abroad as a small sum in the context of the overall welfare bill.

“The payments can only be made while the parents are working in the UK and paying UK national insurance,” he said. “Less than 0.5 per cent of child benefits payments are of his kind.”